If a distortion, such as a short circuit fault, occurs in the power grid, the power network system may get unstable. A temporary short circuit may occur, if for example the power lines are short circuit, power system components are malfunctioning or even brake down.
If a power generating unit detects a voltage dip, the active current fed into the grid is reduced. Particularly, in a prior art fault-ride-through control the active current fed into the power network via a connection grid is reduced as a function of the voltage-dip. This reduced active current is supplied into the connection grid for a comparably short period. Thereafter, the active power is increased to its pre-voltage-dip value.
FIG. 1 shows an exemplary power transmission scenario for a wind park according to the state of the art. Such a wind park comprises a plurality of wind turbines 2 each generating a voltage of e.g. 0.7 kV. The power of the wind turbine 2 is transmitted by a wind turbine connection line 4 to a wind turbine transformer 6 transforming the voltage to e.g. 33 kV. This voltage is supplied by a power line 8 to a wind farm collector grid, i.e. low voltage bus bar, 10. The collector grid voltage value is transformed by a wind farm step-up transformer 12 to a transmission system voltage, e.g. 132 kV. A local load 16 is connected to branch point or first bus 14. From the branch point 14, also referred to as the grid branch radial, a connection line 18, is connected to a transmission network system at a point of common connection also referred to as a second bus 20. Switched capacitors 22 are connected as auxiliary components to the point of common connection 20. These auxiliary components are used for general reactive power control purposes of the system grid voltage.
Network point 28 of a transmission network constitutes an equivalent point of the transmission grid system with a power generating unit 32. This system is connected via interconnection line or first power line 24, the point of common connection 20 and regional transmission line or second power line 26 to a regional transmission network system represented with a second power generating unit 34 and a consumption point 30 of a regional grid system with consumers 36.
In an exemplary scenario electric power is transmitted from the network point or third bus 28 of the transmission network via the point of common connection 20 to the consumption point or fourth bus 30. Further, the power generated from the wind turbines 2 of the wind park is fed into the general transmission grid via the connection grid and connection line respectively, 18 and the point of common connection 20.
In an exemplary fault scenario the regional transmission line 26 is exposed to a severe and damaging short circuit fault and tripped off by the network protection relay. The faulted regional transmission line 26 is heavily damaged and can not be re-connected before it is repaired. The fault scenario causes that the consumption point 30 of the regional grid system no longer can be supplied with power from the wind farm and the grid system located in network point 28. The short circuit causes a severe voltage dip in the entire network system.
With reference to FIGS. 2 to 9 a state of the art fault-ride-through and post fault active power recovery control algorithm is explained.
Particularly, FIGS. 2 to 5 show plots of a simulation of a scenario in which forty-nine wind turbines 2 are connected via the connection line 18 to the point of common connection 20. FIG. 2 shows a plot of a simulation of the relative voltage of the point of common connection 20. A 150 ms severe voltage dip occurs at a point of time of approximately 1 second when the short circuit fault occurs in the regional transmission line 26. When the fault is cleared, the voltage at the point of common connection 20 recovers and after the post fault voltage oscillations have faded out, the voltage in point 20 reaches the same voltage level as before the voltage dip.
FIG. 3 shows a plot of a simulation of the relative voltage of one of the wind turbine connection lines 4. Thus, FIG. 3 represents the relative voltage of the converter of the wind turbine 2. In this state of the art fault-ride-through control the converter reduces the active current fed into the wind turbine connection line 4 by a value depending on the value of the voltage dip.
FIG. 4 shows a plot of a simulation of the total active power from an aggregated wind farm transformer to the low voltage bus bar 10 of the wind farm transformer 12. In the state of the art fault-ride-through control a factor having a value from 0.5 to 1 defines the relationship between the current reduction and the value of the voltage dip. In the current scenario the reduction factor is approximately 0.5. The active power is reduced to 15% of its pre-fault value, as indicated in FIG. 4. The voltage at the wind turbine connection line 4 is about 30% of the pre-fault value as indicated in FIG. 3.
It is to be noted that the active power is ramped back within a comparably small time span of less than 1 second to its pre-fault value. Oscillations of the active power fade out essentially at the pre-voltage dip value.
FIG. 5 shows a plot of a simulation of the reactive power generated by the aggregated wind turbines 2 of the wind farm. The total reactive power from an aggregated wind farm transformer is feed into the low voltage bus bar 10 of the wind faun transformer 12. The post fault total amount of reactive power has changed, since the network grid scenario has changed due to the fact that the system protection relay has disconnected the faulted regional transmission line 26.
It is to be noted that the network remains stable and converges within a comparably small time span after the voltage dip caused by the severe fault in regional transmission line 26. The network system converges although regional transmission line 26 and the consumption point of the regional grid system 30 are disconnected from the point of common connection 20.
The specific fault-ride-through recovery requirements are standardized by each transmission system operator, national service provider and distribution service operator of each country by the so called grid code. This grid code defines how a power generating unit must react in case of a voltage-dip.
FIGS. 6 to 9 show a scenario in which fifty wind turbines 2 are connected via the wind farm transformer 12 and the connection line 18 to the point of common connection 20. FIG. 6 shows a plot of the simulation of the voltage of the common point of connection 20. FIG. 6 corresponds to FIG. 2, wherein FIG. 2 shows a case in which only forty-nine wind turbines form the wind park. FIG. 7 shows a plot of the simulation of the converter voltage of the wind turbine 2 that is feeding power into the wind turbine connection line 4. Thus, FIG. 7 corresponds to FIG. 3, except for the number of connected wind turbines.
FIG. 8 shows a plot of a simulation of the total active power from the aggregated wind farm transformer to the low voltage bus bar 10 of the wind farm transformer 12. Accordingly, FIG. 8 corresponds to FIG. 4, which is a simulation of only forty-nine wind turbines connected to the wind park transformer 12.
Finally, FIG. 9 shows a plot of a simulation of the reactive power fed into the low voltage bus bar 10 of the wind farm transformer 12. Thus, FIG. 9 corresponds to FIG. 5.